The linear and third-order polarizabilities of the light-harvesting antennae of photosynthetic bacteria and green plants are calculated using an equation of motion approach which maps I:he system onto a coupled set of anharmonic excitonic oscillators. The oscillator representation is shown to have several advantages over the traditional picture based on properties of individual global eigenstates of the aggregate; besides a considerable reduction of computational effort, the dynamics of excitations in the two-exciton band is conveniently analyzed in terms of single-exciton Green’s functions and the two-exciton scattering matrix.